1. Field of the Invention
The present invention generally relates to fuel cells operating directly on fuel streams comprising tetramethyl orthocarbonate in which tetramethyl orthocarbonate is directly oxidized at the anode and, more particularly, to solid polymer fuel cells operating directly on liquid fuel streams comprising tetramethyl orthocarbonate.
2. Description of the Related Art
Solid polymer electrochemical fuel cells convert reactants, namely fuel and oxidants, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst is needed to induce the desired electrochemical reactions at the electrodes. Solid polymer fuel cells operate in a range from about 80° C. to about 200° C. and are particularly preferred for portable and motive applications. Solid polymer fuel cells employ a membrane electrode assembly (MEA) which comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrode layers. Flow field plates for directing the reactants across one surface of each electrode substrate are generally disposed on each side of the MEA. The electrocatalyst used may be a metal black, an alloy or a supported metal catalyst, for example, platinum on carbon. The electrocatalyst is typically incorporated at the electrode/electrolyte interfaces. This can be accomplished, for example, by depositing it on a porous electrically conductive sheet material, or “electrode substrate”, or on the membrane electrolyte.
Effective sites on the electrocatalyst are accessible to the reactant, are electrically connected to the fuel cell current collectors, and are ionically connected to the fuel cell electrolyte. Electrons, protons, and possibly other species are typically generated at the anode electrocatalyst. The electrolyte is typically a proton conductor, and protons generated at the anode electrocatalyst migrate through the electrolyte to the cathode.
A measure of electrochemical fuel cell performance is the voltage output from the cell for a given current density. Higher performance is associated with a higher voltage output for a given current density or higher current density for a given voltage output. Another measure of fuel cell performance is the Faradaic efficiency, which is the ratio of the actual output current to the total current associated with the consumption of fuel in the fuel cell. For various reasons, fuel can be consumed in fuel cells without generating an output current, such as when an oxygen bleed is used in the fuel stream (for removing carbon monoxide impurity) or when fuel crosses through a membrane electrolyte and reacts on the cathode instead. A higher Faradaic efficiency thus represents a more efficient use of fuel.
A broad range of reactants have been contemplated for use in electrochemical fuel cells, which reactants may be delivered in gaseous or liquid streams. The oxidant may, for example, be substantially pure oxygen or a dilute oxygen stream such as air. The fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream derived from a suitable feedstock, or a suitable gaseous or liquid organic fuel mixture.
The choice of fuel may vary depending on the fuel cell application. Preferably, the fuel is relatively reactive electrochemically, inexpensive, easy to handle, and relatively safe for the environment. Hydrogen gas is a preferred fuel since it is electrochemically reactive and the by-products of the fuel cell reaction are simply heat and water. However, hydrogen can be more difficult to store and handle than other fuels or fuel feedstocks, particularly in non-stationary applications (e.g., portable or motive). For this reason, liquid fuels are preferred in many applications.
Fuel cell systems employing liquid fuels generally incorporate a reformer to generate hydrogen as required from a liquid feedstock that is easier to store and handle (e.g., methanol). However, the use of a reformer complicates the construction of the system and results in a loss in system efficiency. To avoid using a separate reformer, fuels other than hydrogen may instead be used directly in fuel cells (i.e., supplied unreformed to the fuel cell anodes). Inside the fuel cell, a fuel mixture may be reacted electrochemically (directly oxidized) to generate electricity or instead it may first be reformed in-situ (internally reformed), as in certain high temperature fuel cells (e.g., solid oxide fuel cells). After being internally reformed, the fuel is then electrochemically converted to generate electricity. While such fuel cell systems may employ fuels that are easier to handle than hydrogen, and without the need for a separate reformer subsystem, generally hydrogen offers fundamental advantages with regard to performance and the environment. Thus, improvements in these areas are desirable in order for internally reforming and direct oxidation fuel cell systems to compete more favorably to hydrogen-based systems.
A direct methanol fuel cell (DMFC) is a type of direct oxidation fuel cell that has received much attention recently. A DMFC is generally a liquid feed solid polymer fuel cell that operates directly on an aqueous methanol fuel mixture. The anode and cathode reactions in a direct methanol fuel cell are shown in the following equations:Anode reaction: CH3OH+H2O6H++CO2+6e−Cathode reaction: 3/2O2+6H++6e−3H2OOverall reaction: CH3OH+3/2O2CO2+2H2O
There is often a problem in DMFCs with substantial crossover of methanol fuel from the anode to the cathode side through the membrane electrolyte. The methanol that crosses over then reacts with oxidant at the cathode and cannot be recovered, resulting in significant fuel inefficiency and deterioration in fuel cell performance. To reduce crossover, very dilute solutions of methanol (e.g., about 5% methanol in water) are typically used as fuel streams in DMFCs. Unfortunately, such dilute solutions afford only minimal protection against freezing during system shutdown in cold weather conditions, typically down to about −5° C.
In order to overcome the disadvantages of aqueous methanol fuel mixtures, efforts have been made to develop alternative liquid fuels for direct use in direct liquid feed fuel cells (DLFFCs). For example, published PCT WO 96/12317 discloses alternative liquid fuels for use within DLFFCs, including dimethoxymethane, trimethoxymethane, and trioxane. Additionally, in published PCT WO 99/44253, the direct use of dimethyl ether (DME) in a DLFFC is disclosed. Like methanol, these fuels can be oxidized at the fuel cell anode to form carbon dioxide and water at a rate that provides satisfactory fuel cell performance.
Recently, significant efforts have been made to develop micro direct liquid feed fuel cells (MDLFFCs) for low-power and portable applications, such as cellular phones. For simplification and to reduce size, a MDLFFC preferably contains no air or fuel pumps, with the reactants being passively supplied to the electrodes instead. Conventional methods for feeding fuel to the fuel cell stack in a MDLFFC include the use of gravitational flow, capillary flow, natural diffusion and/or natural convection of the fuel from a fuel reservoir to the anodes. Fuel cell reaction products are transported away from the anode and cathode surfaces to the atmosphere by means of natural diffusion and/or convection. For example, Mench et al. (“Design of a Micro Direct Methanol Fuel Cell (μDMFC)”, International Mechanical Engineering Congress and Exposition, New York, N.Y., Nov. 11-16, 2001) discloses a micro DMFC which uses gravitational and capillary forces to feed fuel to its anodes, having a total volume of 1 cm3 and expected to have a power density of about 1 W/cm3. Similarly, Narayanan et al. (“Design and Development of Miniature DMFC Power Sources for Cellular Phone Applications”, 2000 Fuel Cell Seminar Abstracts, Portland, Oreg. Oct. 30-Nov. 2, 2000, pp. 795-798) discloses a micro DMFC fed by diffusion that yields a power density of about 8 mW/cm2.
While significant advances have been made in this field, there remains a need in the art for new and effective fuels that provide comparable performance to an aqueous methanol fuel solution in both DLFFCs and MDLFFCs. The present invention fulfills these needs and provides further related advantages.